Low Probability of Intercept (LPI) radars are military radars which are designed for the modern electronic combat environment. More or less successfully, they try to avoid being detected by ELINT sensors by using any combination of the features outlined in this Entry.

Multistatic Radar

Common radars are monostatic - that is, their transmitter and receiver are located in the same place. An ELINT sensor can only detect transmitters and once the transmitter has been located, the whole radar can be engaged by jamming or by other means such as artillery or anti-radiation missiles.

Bistatic radars have the transmitter and receiver at different places. Therefore, the transmitter may be located, but destroying the transmitter won't lead to much damage as the receiver and signal-processing equipment makes up the more expensive part of a radar. It doesn't make sense to direct a jammer against the transmitter because only a receiver can be susceptible to jamming. A special case of bistatic radar is the pair that is made up by a target illumination radar and the seeker head of a radar homing missile. Attempting to jam the illuminator won't affect the seeker-head. Even worse, doing so would provide it with a beacon signal.

Multistatic radars are the extension of bistatic ones. They've got a single transmitter but have several receivers distributed over an area of interest. The receivers deliver their findings to an evaluation centre where powerful computers are busy correlating the results and putting together an air picture. Transmitters are much cheaper than all this equipment and if a transmitter was to be destroyed, it would be fairly easy to replace it.

Ultra-low Sidelobe Antennae

Ordinary radar antennae feature sidelobes which are weaker by a factor of, for example, 100 times than the main lobe. That is, they are giving away their position not only in the intended direction but also to anybody who cares to listen from somewhere else. Ultra-low sidelobe antennae are designed to change that by featuring sidelobe levels in the ranges of one ten-thousandth of the power of the main lobe and even less. Thus, the chances of intercepting a signal from a position outside the main beam are significantly reduced.

Ultra-wide Band Signals

Long pulse duration means that a given amount of energy is distributed over time. Ultra-wide band signals distribute a given amount of energy over frequency, with the same effect of hardening an ELINT receiver's task of detecting them. The radar itself knows which signal it is looking for, and the receiver circuitry has been matched to the signal properties. This knowledge gives the radar receiver a distinct advantage over an ELINT receiver.

Long Pulse, Low Power

A radar receiver needs a distinct amount of energy in order to detect a radar return. This energy can be contained in a short but powerful pulse, in a weak pulse with correspondingly longer duration, or ultimately in a continuous wave signal with even less power. ELINT receivers rely on some minimum ratio of power to noise in order to identify a radar signal. Low power signals are designed to evade detection by lowering this ratio and trying to hide within the noise.

No Power At All: Passive Radar

A passive radar like Silent Sentry [Note: This is a PDF document, requiring the free Adobe Acrobat Reader to be installed on your machine. The software can be downloaded for free from Adobe] is a multistatic radar - without the transmitter. The receivers pick up reflections that are caused by background illumination provided by commercial TV or radio stations. Enormous computer power is engaged to correlate them against a master copy that was received via some direct propagation path and to calculate target flightpaths and positions. As this type of radar doesn't have any transmitter, it is obvious that an enemy's ELINT assets are confronted with the problem of finding something that doesn't actually exist.

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